LTC3521



3521f

Buck Input Capacitor Selection

switch and is the supply pin for the IC’s internal circuitry.

It is recommended that a low ESR ceramic capacitor with

a value of at least 4.7µF be used to bypass this pin. The

capacitor should be placed as close to the pin as possible

and have a short return to ground.

Buck Output Voltage Programming

The output voltage is set by a resistive divider, according

to the following formula:

V

V

R

R

OUT2 3

0 6 1

2

1

,

.

=

+









Theexternaldividerisconnectedtotheoutput,asshownin

Figure 3. It is recommended that a feedforward capacitor,

noise immunity of the feedback node. Table 3 provides

the recommended resistor and feedforward capacitor

combinations for common output voltage options.

Table 3. Buck Resistor Divider Values

R1

R2

0.6V

–

0

–

0.8V

200k

69.8k

22pF

1.0V

118k

80.6k

22pF

1.2V

100k

102k

22pF

1.5V

78.7k

121k

22pF

1.8V

68.1k

137k

22pF

2.7V

63.4k

226k

33pF

3.3V

60.4k

274k

33pF

Buck-Boost Output Voltage Programming

The buck-boost output voltage is set by a resistive divider

according to the following formula:

V

V

R

R

OUT1

0 6 1

2

1

=

+









.

The external divider is connected to the output, as shown

in Figure 4. The buck-boost converter utilizes voltage

mode control and the value of R2 plays an integral role

in the dynamics of the feedback loop. In general, a larger

value for R2 will increase stability and reduce the speed of

the transient response. A smaller value of R2 will reduce

stability but increase the transient response speed. A good

starting point is to choose R2 = 1MΩ, then calculate the

required value of R1 to set the desired output voltage ac-

cording to the above formula. If a large output capacitor

is used, the bandwidth of the converter is reduced. In

such cases R2 can be reduced to improve the transient

response. If a large inductor or small output capacitor is

utilized, the loop will be less stable and the phase margin

can be improved by increasing the value of R2.

Buck-Boost Inductor Selection

To achieve high efficiency, a low ESR inductor should be

utilized for the buck-boost converter. The inductor must

haveasaturationratinggreaterthantheworstcaseaverage

inductor current plus half the ripple current. The peak-to-

peakinductorcurrentripplewillbelargerinbuckandboost

mode than in the buck-boost region. The peak-to-peak

inductor current ripple for each mode can be calculated

applicaTions inForMaTion

LTC3521

GND

FB3

R1

3521 F03

R2

FB2

R1

R2

Figure 3. Setting the Buck Output Voltage

LTC3521

GND

FB1

R1

3521 F04

R2

Figure 4. Setting the Buck-Boost Output Voltage